BANGKOK, Thailand -- Creditors of Thailand's biggest corporate debtor, Thai Petrochemical Industry PCL, suffered a setback when an objection by the company's founder stalled a long-awaited vote to approve a restructuring of its $3.7 billion debt.

Creditors had assembled on Monday expecting to vote on a five-year debt plan drawn up by Effective Planners Ltd., a subsidiary of Australian accounting firm Ferrier Hodgson. The plan provides for creditors to swap $800 million in outstanding interest for equity. However, officials...